1. Field of the Invention
My invention relates to the field of digital communications, and more specifically to the communication of binary information by means of a serial stream of discrete signals.
2. Background of the Invention
Information is commonly processed and stored in binary form, and often the methods and machines used make is advantageous to group bits together into "words." Different processes and storage devices, however, frequently use different word lengths, i.e., number of bits per word. For example, consider the process of transferring data from one computer to another computer at a different location. First, it is desirable for the human operators to be able to communicate. This can be done efficiently using six bit words to represent the alphanumeric keyboard characters. Next, the destination computer must be sent information such as where to store the data and how many words to expect. It is important that this "control" information be distinguishable from the data, for these control words are to be acted upon, but not stored. Finally, there is the data itself, which must not--regardless of its contents--invoke any control actions in the communication equipment or the computer.
For most efficient operation, the word length of the transmitted words should match that of the computer. But even when restricted to the most common computers, word lengths of 8, 12, 16, 32, and 48 must be considered. Moreover, with the inclusion of each additional word length, the problem of distinguishing the various types of information is exacerbated.
In order to accommodate widely varying word sizes, many current systems compromise on a fixed word length of eight bits, with longer groups being sent as more than one word. The result is that each information group must first be subdivided at the transmitting end, then reassembled at the receiving end. Keyboard characters end up being sent one per word, with two bits wasted.
In order to solve the problem of distinguishing between operator messages, control information, and the data, a rigid format or protocol is often adopted. Most protocols require special characters such as "start-of-header," "start-of-text," and "end-of-text." As a consequence, either circuit complexity or processing overhead is significantly increased. In addition, if the bit patterns corresponding to the special characters randomly occur in the data, there are still more complications.
In some present systems, a continuous, synchronous bit stream--devoid of word delimiters--is transmitted. While this technique is efficient for sending information of varying word lengths, it requires blocking off the stream into "frames." Frame marker bits are inserted periodically, along with control information. The shortcoming of this technique is that a single dropped bit can invalidate all the information within a frame.